The present invention relates to utilizing a beam of light to detect the presence of objects, and more particularly to optical sensing equipment which employ a plurality of photodetectors to receive the light which has been reflected by an object to be detected.
In manufacturing operations, it is desirable to detect the presence of an object moving down a conveyor. This enables material handling equipment to direct the object safely between conveyor sections or to a work station. It also is desirable to detect when objects become jammed along the conveyor.
An optical detector system often is used for these purposes. One type of detection apparatus, referred to as a retro-reflective system places an emitter-detector assembly on one side of the conveyor and a reflector on the opposite side. A beam of light is sent from the emitter across the conveyor to the reflector and then returns back across the conveyor to the detector. An object, moving along the conveyor, interrupts the beam of light, thereby providing an indication of the presence of the object. Retro-reflective systems have the disadvantage of requiring installation of a reflector on the opposite of the conveyor. Installation of the reflector in many situations is difficult or interferes with other operations being performed along the conveyor. Therefore, it is desirable to utilize an object detector apparatus that does not require devices on both side of the conveyor.
In response, sensing systems have been developed which detect the reflection of the light beam from objects moving alone the conveyor. However, such systems must address several potential problems. First, the reflectivity of the objects vary greatly from very specular in nature to ones that are very diffuse. In addition, black objects naturally absorb more light than white objects. The circuitry that processes the signal from the light detector can be designed with a relatively high sensitivity to detect low reflectivity objects. However, that high sensitivity often results in the signal processing circuitry being saturated in response to light from highly reflective objects.
In addition, high sensitivity sensing circuits can falsely respond to highly reflective objects on the opposite side of the conveyor. For example, a shiny metal object being transported on a cart next to the conveyor system can reflect enough light back to the photodetectors to be erroneously interpreted as an object moving down the conveyor. Therefore, it is desirable to have a detector system that has a relatively high sensitivity and a sharp cutoff at a distance equal to the far side of the conveyor.
FIG. 1 depicts a prior detection system of this type. In this system, an emitter 10 transmits a beam of light across the conveyor. One ray 12 of that light beam is illustrated passing through an output lens 14. An object 16 reflects the ray 12 through another lens 18 onto a detector assembly 20. The detector assembly 20 has an near detector 22 that receives light from objects which are relatively close to the detector assembly and has a far detector 24 that receives light from objects which farther away from the detector assembly. Note that the objects usually move in a direction that is orthogonal to the plane of the drawing. The farther an object is from the emitter 10 the smaller the angle of the reflected ray 12. For example, the reflected beam from object 16 strikes the near detector 22, whereas the reflected ray from a more distant object 26, beyond a given cutoff distance 28 from the emitter, strikes the far detector 24 and not the near detector 22.
It should be understood that the emitter 10 produces a beam of radiation comprising numerous rays. Thus, when the entire beam is reflected from an object, some of the rays may strike the near detector 22 and other reflected rays may strike the far detector 24. If the object is within the cutoff distance 28 from the emitter, a greater amount of reflected light will strike the near detector 22 than the far detector 24. Conversely, when the object is beyond the cutoff distance 29, a greater amount of light is reflected onto the far detector 24 than onto the near detector 22. The output signal produced by a detector corresponds to the amount of light which impinges that detector. Thus, by comparing the two detector signals, the object detection apparatus is able to distinguish an object moving along the conveyor from objects beyond the conveyor.
This dual detector system does well when a diffuse object fully blocks the beam of light from the emitter. However, when an out of range object blocks only a portion of the emitted beam or is specular, light from that out of range object can be falsely interpreted as being from an object on the conveyor because the near detector 22 may receive more light than the far detector 24.
An object detection system has an emitter which produces a light beam that will be reflected by objects to be detected. First and second photodetectors are located on one side of the emitter and a third photodetector is located on an opposite side of the emitter. The first, second and third photodetectors respectively produce first, second and third signals in response to being struck by the emitter light that is reflected by an object.
The three photodetectors have separate fields of view that are aimed so that the second signal from the second photodetector will be greater than the combined signals from the first and third photodetectors when an object is within a given distance of the emitter. This given distance referred to as the cutoff distance defines the sensing range of the object detection system. When an object is beyond the cutoff distance, the combined first and third signals from the first and third photodetectors will be greater than the second signal. Therefore, a processing circuit is able to determine whether an object is within the cutoff distance by arithmetically combining the three photodetector signals. The three photodetectors are aimed so that even a specular object that is beyond the cutoff distance will not produce signals that are falsely interpreted as coming from an object within range.
In the preferred embodiment, the three photodetectors are photodiodes and the first and second photodetectors are connected inversely in parallel. This results in the signal from the first photodetector being subtracted from the signal from the second photodetector at a first input node to which both photodetectors are connected. An amplifier has an input coupled to the first input node and an output coupled to a summing node. The third photodetector is coupled to an input of another amplifier that has an output also coupled to the summing node. Thus the signal produced at the summing node indicates presence of an object within range of the object detection system.